This invention relates to the manufacture of finely divided boehmite, (sometimes called alpha alumina monohydrate), from alumina trihydrate, amorphous alumina, less finely divided boehmite and the like.
In the production of ceramic forms of alumina with a very fine crystalline size of less than one micron, it is highly desirable to use as a starting material a similarly finely divided form of the precursor material. This is particularly true when the process used is a sol-gel process such as is described in, for example U.S. Pat. Nos. 4,314,827 or 4,623,364. Such patents use a sol of alumina monohydrate which is then gelled either by reduction of water content or more usually by peptization using an acid. This gelled form may then be dried, comminuted and sintered at relatively low temperatures to give alpha alumina structures with very fine and uniform particle sizes. The above process may be carried out in the presence of modifying agents such as magnesia, zirconia, silica, titania, chromia or compounds that break down to such oxides under sintering conditions, and/or seed materials that favor the formation of the alpha alumina form. Such seed materials can be any one of those materials that are stable under the reaction conditions and which have the same crystal structure as the target alpha alumina and similar lattice parameters to those of alpha alumina. The preferred seed material is of course alpha alumina itself but other seed materials within the above definition such as alpha ferric oxide, added ab initio or generated in situ, can be used in many circumstances.
As indicated above it is highly advantageous in the operation of such a process that the boehmite dispersion be as fine as possible and as free as possible of agglomerates which tend to convert as such and produce similar agglomerates in the alpha form. It is found that such agglomerates are associated with a reduction in the properties of the final products. For example, abrasive grits made of such material are often less effective than those in which the crystal size is uniformly microcrystalline.
It is apparent therefore that there is substantial advantage in securing a feedstock that has the uniform microfine crystal structure referred to above. One method of obtaining such products is described in U.S. Pat. No. 4,117,105 in which an alumina trihydrate is first calcined to increase its surface area through a partial dehydration and then autoclaved to achieve crystallization and rehydration. The result is apparently to produce a product with a crystallite size smaller than that of the starting material.
Another process for producing a water dispersible alumina from a hydrolysate of an aluminum alkoxide is described in U.S. Pat. No. 4,676,928. In this process the alumina slurry is aged at a pH of 5 to 9 at a temperature above 70.degree. C. for enough time to convert the greater part to a colloidal sol.
Other processes for the production of alumina in a sol form are described in U.S. Pat. Nos. 2,590,833; 2,787,522; 2,915,473; 3,357,791 and 3,966,893.
However none of the above methods are capable of converting an alumina monohydrate precursor having a substantial degree of particulate inhomogeneity into a product with essentially uniform, ultrafine, (i.e. less than 1000 angstroms), particles that are particularly suitable for the production of alpha alumina by a sol-gel route. The present invention provides such a process and thus makes available to the maker of high grade alpha alumina products, or the catalyst monolith producer where alpha alumina is not required, a feedstock that had previously been unacceptable on account of the presence of agglomerated particles.